Macrocyclic ligands isomerism

Macrocycles history, 21 metal complexes protonation, 423 Macrocyclic ligands isomerism, 204 Magnesium gravimetry, 528... 

Apart from this one-reaction type, the routine use of metal template procedures for obtaining a wide range of macrocyclic systems stems from 1960 when Curtis discovered a template reaction for obtaining an isomeric pair of Ni(n) macrocyclic complexes (Curtis, 1960). Details of this reaction are discussed later in this chapter. The template synthesis of these complexes marked the beginning of renewed interest in macrocyclic ligand chemistry which continues to the present day. 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

The coordination chemistry of macrocyclic ligands has been extensively studied and aspects of isomerism have been considered in numerous systems.241 Methods whereby two diastereomers of complexes of tetra- N-methylcyclam may be isolated have been discussed previously.184 This, however, is a relatively simple system and it is usually necessary to consider isomerism due to the presence of asymmetric atoms in the chelate arms, as well as that due to asymmetric donor atoms that may be rendered stable to inversion by coordination. An example of a system exhibiting this level of complexity is afforded by the nickel(II) complexes of the macrocyclic ligands generated by reduction of the readily prepared macrocycle (46). These ligands contain two asymmetric carbon atoms and four asymmetric nitrogen atoms but, because AT-inversion is rapid, it is conventional to consider that only three separable stereoisomers exist. There is an enantiomeric pair, (47a) and (47b), which constitutes the racemic isomer (R, R ), and an achiral (R, S ) diastereomer (47c), the meso isomer. 

The worksheet in Figure 23-3 illustrates a case of a reaction so slow that it was necessary to use the Solver to find the final absorbance reading. The unstable czs-octahedral isomer of the nickel(II) complex of the macrocyclic ligand cyclam (1,4,8,11-tetraazacyclotetradecane) isomerizes to the planar complex... 

The unstable ds-octahedral isomer of the nickel(ll) complex of the macrocyclic ligand 13aneN4 (1,4,7,10-tetraazacyclotridecane) isomerizes to an intermediate planar isomer, which then converts to the stable planar isomer... 

The reaction of planar Ni ([14]aneN4) + represented as shown in (2.10) with a number of bidentate ligands (XY) to produce c -octahedral Ni ([14]aneN4) XY + is first-order in nickel complex and [OH ] and independent of the concentration of XY.In the preferred mechanism, the folding of the macrocycle (base-catalyzed tmns — cis isomerization) is rate determining, and this is followed by rapid coordination of XY ... 

---